The present invention relates to a magnetic recording medium such as a floppy disk and a recording/reproducing method. Specifically, the present invention relates to a magnetic recording medium suitable for the positioning of a read/write head using an optical detection means and a recording/reproducing method using the magnetic recording medium.
A large amount of floppy disks has been used as a secondary storage medium to store data for computers and word processors. With typical floppy disk systems, positioning of a magnetic head is performed by an open-loop control using a stepping motor. This restricts further increase of a track density.
In recent years, magnetic recording media have been proposed that use optical servo techniques to achieve much higher track densities thereon. More specifically, an optical pattern is formed on a magnetic recording medium by means of forming a certain area having different optical characteristics from a reference area. A read/write head has an optical sensor integrally provided therewith to read the optical pattern. The sensed data is used as a tracking servo signal for positioning the head. This technique offers positioning of the head by a closed-loop control even with the floppy disk systems. Such fine positioning of the head permits realization of track densities higher by one digit than conventional ones.
The optical pattern for use in generating the tracking servo signal may be formed in various ways. For example, Japanese Patent Laid Open No. 2-31378 disclosed a method of forming a groove on a surface of a magnetic layer by means of laser or the like. International Application No. PCT/WO94/11867 proposes a method of adding dyes to a magnetic layer in the disk and focusing a beam on a certain spot thereon to cause reaction such as decomposition of the dyes at that spot, thereby changing the optical characteristics thereof. The beam may be a light beam such as a laser beam, an ultraviolet radiation, a strobo light (electronic flash), or an electron beam.
In the tracking servo system using such optical sensing means, a transparency of the magnetic recording medium is an important factor.
More specifically, an incident beam is focused on the magnetic recording medium. A portion of the incident beam is reflected from the medium and another portion is transmitted therethrough. The optical sensor typically detects either the reflected beam or the transmitted beam. The tracking servo signal is obtained according to a difference in light reflectance or light transmittance between the reference area and the area having the different optical characteristics. A problem occurs particularly when the optical sensor detects the transmitted light beam that the difference in light transmittance becomes extremely small when the magnetic recording medium has a low transparency, causing incorrect tracking. In tracking with the transmitted light beam, the light transmittance is approximately in proportion to the servo signal output. Accordingly, the transparency of the magnetic recording medium is the most important challenge that affects results of servo tracking on the magnetic recording medium.
The conventional floppy disks have magnetic layers containing carbon black in order to avoid electrification. While the carbon black contributes to imparting electrical conductivity to the layers, it badly affects the transparency thereof. The transmittance to light having a wavelength of 830 nm is as low as approximately 5%.
With this respect, it has been proposed to achieve satisfactory level of electrical conductivity without using the carbon black. To this end, an intermediate layer is provided between a magnetic layer and a non-magnetic support for supporting the magnetic layer. Conductive metals, metal compounds, or resins are contained in the intermediate layer to impart the electrical conductivity to the magnetic recording medium. The resultant magnetic recording medium has both the electrical conductivity and the transparency.
However, the present inventors hsve found, as a result of careful studies, that the magnetic recording medium of the type described still has a serious problem. The magnetic recording medium containing substantially no carbon black is inferior in surface roughness and has a low running resistance. It is thus difficult to satisfy fundamental characteristics required for the magnetic recording medium.